This invention relates to a method for manufacturing a semiconductor device including a patterning process based on immersion exposure.
A method for manufacturing a semiconductor device typically includes numerous processes for depositing a plurality of materials, including a subject film, on a semiconductor substrate and patterning them into desired patterns. In the patterning process, a film of photosensitive material, called resist, is formed on the subject film, and the resist film is subjected to selective exposure using a mask (reticle). Subsequently, the exposed or unexposed portion of the resist film is removed by development to form a resist pattern, which is further used as a mask to process the subject film.
Commonly used exposure light sources include ultraviolet lasers such as KrF excimer lasers and ArF excimer lasers. However, with the miniaturization of integrated circuit patterns, the required resolution is falling below the wavelength of such ultraviolet light. Thus, exposure process margins such as exposure amount margin and focus margin are becoming insufficient.
Studies to increase the numerical aperture (NA) toward resolution enhancement have now led to the immersion exposure technique in which exposure is performed through a liquid filled between the resist film surface and the projection lens. In such immersion exposure, the immersion liquid is in contact with the resist film surface, and the number of defects depends on the contact angle. For instance, in scan exposure, if the contact angle of the immersion liquid in the scan direction (advancing contact angle) is too high, defects due to air inclusions are generated. On the other hand, if the contact angle of the immersion liquid on the opposite side of the scan direction (receding contact angle) is low, the immersion liquid is left behind on the resist and causes defects due to remaining liquid.
As a resist for immersion lithography, JP-A-2009-004478(Kokai), for instance, proposes use of a topcoat-less resist, that is, a resist requiring no protective film between the resist film and the immersion liquid. When the topcoat-less resist is used, the concentration distribution of resist components including a water-repellent additive in the film thickness direction varies with the wafer rotation speed during spin coating of the resist, causing variation in the contact angle of the immersion liquid in contact with the resist surface. Furthermore, the film thickness of the resist film is controlled by the wafer rotation speed. Hence, if the wafer rotation speed is changed with the design film thickness, there is concern about variation in the contact angle of the resist surface and generation of defects during immersion exposure. Thus, it is desirable to maintain the contact angle of the resist surface at a desired value irrespective of the design film thickness of the resist film, or the wafer rotation speed.